The present invention relates to power steering for automobiles with a mechanical drive connection between a steering device, particularly a steering wheel, and a steering gear arrangement for steering of vehicle wheels, a servo motor coupled with the steering gear arrangement and a connection for separating the drive connection. The servo motor is controllable over a control valve which, in turn, can be controlled by a control system as a function of a difference between a first value, correlated with the steering angle of the vehicle wheels, and at least one second value, correlated with the adjusting travel of the steering device. A servo valve additionally is connected with the drive connection and analogous thereto, can be controlled by the forces or moments transferred by the drive connection, over which servo valve the servo motor can be connected in a controllable manner with delivery and low-pressure connections. Thereby, the servo motor produces a servo power reducing the actuation force necessary at the steering device and the servo motor being connected through a reversing valve arrangement either to the motor connections of the control valve or to the motor connections of the servo valve.
Such power steering arrangements are known from JP-A-40 19271. Steering arrangements of this type have two operating states. In one operating state, a mechanical forced connection is provided between the steering device and the steered vehicle wheels whereby, as a function of the forces transferred between the steered vehicle wheels and the steering device, a servo valve is controlled which, in turn, actuates a hydraulic servo motor. In this operating state, the steering therefore works like conventional hydraulic power steering. In the other operating state, the steering device is disconnected from the steered vehicle wheels, and the servo motor is controlled by a control system as a function of a difference between a first value correlated with the steering angle of the steered wheels of the vehicle and a second value correlated at least with the adjusting travel of the steering device. In this state, the steering therefore works according to the "steer by wire" concept.
To switch between the two operating states at need, a reversing valve arrangement connects the servo motor either with the motor connections of the servo valve or with the motor connections of the servo valve actuated by the control system. Therefore, in the event that the control system malfunctions, it is possible to switch over to the operating method of the conventional power steering system.
In JP-A-40 19271, the valve arrangement, which is provided for the above-cited switching, normally is in the position provided for the "steer by wire" operating state. The reversing valve arrangement is switched hydraulically from this normal position when the servo valve is actuated with the mechanical forced connection switched on between the steering device and the steered wheels of the vehicle. The servo motor is built in a special manner for this purpose. There are objections to this concept for safety reasons.
A vehicle steering system is described in WO 90/12723 in which the mechanical forced connection, provided for emergency purposes between a steering wheel and the wheels steering the vehicle, is normally separated. In this normal state, a first hydraulic adjusting unit, serving to adjust the steering of the wheels steering the vehicle and a second hydraulic adjusting unit, serving to produce a manual force perceptible at the steering wheel, are actuated. Both adjusting units are controlled by electronics. These control electronics, on one hand, actuate the first hydraulic adjusting unit assigned to the steering wheels of the vehicle in accordance with signals from a nominal value transmitter actuated by the steering wheel for the desired angle of steering. On the other hand, the second hydraulic adjusting unit, assigned to the steering wheel and serving for production of manual power, is actuated by the control electronics, whereby various types of parameters can be taken into consideration. The control electronics create the possibility of adjusting the ratio between the change in steering angle of the wheels steering the vehicle and the change in angle of rotation of the steering wheel to a value desired for good handling of the vehicle, without having to demand an excessive expenditure of force on the part of the driver, because the driver need only overcome the resistance of the second adjusting unit serving to adjust the manual force. The actual forces necessary for the steering adjustment of the wheels steering the vehicle are applied by the first adjusting unit. In the final analysis, an easy, but nevertheless direct steering is thus created, in which a relatively small shift in rotation of the steering wheel leads to a clear change in angle of steering of the wheels steering the vehicle. only in emergency cases is a mechanical forced connection switched in between the steering wheel and the wheels steering the vehicle. Comparatively indirect steering, i.e., a relatively large shift in rotation of the steering wheel leads to only a relatively small change in the angle of steering of the steered wheels of the vehicle, must be accepted in this system for limiting the manual force necessary.
A conventional power steering is described in DE 44 09 928 A1, in which the steering wheel is constantly connected mechanically with the steered wheels of the vehicle. A pump serving as a source of hydraulic pressure is combined with a hydraulic pressure accumulator which can be charged by the pump as required or maintained at a specified boost pressure. The pressure accumulator is connected over a pressure line controlled by a check valve to a pressure connection of the servo valve. This pressure connection is also connected with the delivery side of the pump over a control line, in which a throttle is located. In phases, in which there is no need for servo support, the servo valve is in an open central position, in which the above-cited pressure connection is connected with both the servo motor connections and a low-pressure connection. Thereby, the servo valve is connected with a relatively low-pressure hydraulic reservoir. The hydraulic stream flowing over the above-mentioned control line from the pump to the servo valve is fed to the reservoir over the servo valve, as long as the servo valve is in the state with its open center. If, now, during a steering maneuver, the servo valve is increasingly shifted, increasing throttle resistance arises between the pressure connection and the low-pressure connection of the servo valve with the consequence that the hydraulic pressure in the control line increases sharply. This pressure is transmitted to a control input of the control valve between the pressure accumulator and the servo valve with the consequence that the control valve is now opened and the pressure line released, that is, the pressure connection of the servo valve is connected with the pressure accumulator. Thus, a sufficiently large hydraulic stream, supplied under high pressure from the pressure accumulator and/or pump, is available for producing a larger servo force.
An object of the present invention is to ensure a high degree of safety for a power steering system of the above-described type.
This object has been accomplished owing to the fact that the servo motor can be connected over normally-open reversing valves with the motor connections of the servo valve and over normally closed reversing valves with the motor connections of the control valve and these reversing valves can be controlled by the control system into their other position only if the control system is working properly.
In the present invention, therefore, the reversing valve arrangement normally is in the position necessary in the event of a malfunctioning of the control system, i.e., when the steering is supposed to work as a conventional hydraulic power steering system. Only when the control system is operating perfectly can the reversing valve arrangement assume the other position for the "steer by wire" operating mode, provision having been made for the control system to carry out this valve reversal.
The present inventive system excels due to the greatest degree of safety, since it is possible, without anything further, for the connection to switch without delay into its closed state and thus produce the mechanical connection between steering device and the steering wheels of the vehicle as soon as errors are detected in the control system which, in a manner known in principle, can constantly check itself for error-free function.
With the connection open, in principle, certain conversion relationships can be realized between the steering device and the steering wheels of the vehicle. In addition, when the nominal values for the steering angle of the steering wheels of the vehicle are specified, disruptive forces affecting the vehicle, such as crosswinds or other parameters, such as, for example, yaw rate, can be taken into consideration.